Iontophoresis, according to Dorland's Illustrated Medical Dictionary, is defined to be "the introduction, by means of electric current, of ions of soluble salts into the tissues of the body for therapeutic purposes." Iontophoretic devices have been known since the early 1900's. British patent specification No. 410,009 (1934) describes an iontophoretic device which overcame one of the disadvantages of such early devices known to the art at that time, namely the requirement of a special low tension (low voltage) source of current which meant that the patient needed to be immobilized near such source. The device of that British specification was made by forming a galvanic cell from the electrodes and the material containing the medicament or drug to be delivered transdermally. The galvanic cell produced the current necessary for iontophoretically delivering the medicament. This ambulatory device thus permitted iontophoretic drug delivery with substantially less interference with the patient's daily activities.
More recently, a number of United States patents have issued in the iontophoresis field, indicating a renewed interest in this mode of drug delivery. For example, U.S. Pat. No. 3,991,755 issued to Vernon et al; U.S. Pat. No. 4,141,359 issued to Jacobsen et al; U.S. Pat. No. 4,398,545 issued to Wilson; and U.S. Pat. No. 4,250,878 issued to Jacobsen disclose examples of iontophoretic devices and some applications thereof. The iontophoresis process has been found to be useful in the transdermal administration of medicaments or drugs including lidocaine hydrochloride, hydrocortisone, fluoride, penicillin, dexamethasone sodium phosphate, insulin and many other drugs. Perhaps the most common use of iontophoresis is in diagnosing cystic fibrosis by delivering pilocarpine salts iontophoretically. The pilocarpine stimulates sweat production; the sweat is collected and analyzed for its chloride content to detect the presence of the disease.
In presently known iontophoretic devices, at least two electrodes are used. Both of these electrodes are disposed so as to be in intimate electrical contact with some portion of the skin of the body. One electrode, called the active or donor electrode, is the electrode from which the ionic substance, medicament, drug precursor or drug is delivered into the body by iontophoresis. The other electrode, called the counter or return electrode, serves to close the electrical circuit through the body. In conjunction with the patient's skin contacted by the electrodes, the circuit is completed by connection of the electrodes to a source of electrical energy, e.g., a battery. For example, if the ionic substance to be delivered into the body is positively charged (i.e., a cation), then the anode will be the active electrode and the cathode will serve to complete the circuit. If the ionic substance to be delivered is negatively charged (i.e., an anion), then the cathode will be the active electrode and the anode will be the counter electrode.
Alternatively, both the anode and cathode may be used to deliver drugs of opposite charge into the body. In such a case, both electrodes are considered to be active or donor electrodes. For example, the anode can deliver a positively charged ionic substance into the body while the cathode can deliver a negatively charged ionic substance into the body.
It is also known that iontophoretic delivery devices can be used to deliver an uncharged drug or agent into the body. This is accomplished by a process called electroosmosis. Electroosmosis is the transdermal flux of a liquid solvent (e.g., the liquid solvent containing the uncharged drug or agent) which is induced by the presence of an electric field imposed across the skin by the donor electrode.
Furthermore, existing iontophoresis devices generally require a reservoir or source of the beneficial agent (which is preferably an ionized or ionizable agent or a precursor of such agent) to be iontophoretically delivered or introduced into the body. Examples of such reservoirs or sources of ionized or ionizable agents include a pouch as described in the previously mentioned Jacobsen U.S. Pat. No. 4,250,878, or a preformed gel body as described in Webster U.S. Pat. No. 4,383,529 and Ariura et al. U.S. Pat. No. 4,474,570. Such drug reservoirs are electrically connected to the anode or the cathode of an iontophoresis device to provide a fixed or renewable source of one or more desired agents.
More recently, iontophoretic delivery devices have been developed in which the donor and counter electrode assemblies have a "multi-laminate" construction. In these devices, the donor and counter electrode assemblies are each formed by multiple layers of (usually) polymeric matrices. For example, Parsi U.S. Pat. No. 4,731,049 discloses a donor electrode assembly having hydrophilic polymer based electrolyte reservoir and drug reservoir layers, a skin-contacting hydrogel layer, and optionally one or more semipermeable membrane layers. In addition, Ariura et al, U.S. Pat. No. 4,474,570 discloses a device wherein the electrode assemblies include a conductive resin film electrode layer, a hydrophilic gel reservoir layer, and aluminum foil conductor layer and an insulating backing layer.
The drug and electrolyte reservoir layers of iontophoretic delivery devices have been formed of hydrophilic polymers. See for example, Ariura et al, U.S. Pat. No. 4,474,570; Webster U.S. Pat. No. 4,383,529 and Sasaki U.S. Pat. No. 4,764,164. There are several reasons for using hydrophilic polymers. First, water is a the preferred solvent for ionizing many drug salts. Secondly, hydrophilic polymer components (i.e., the drug reservoir in the donor electrode and the electrolyte reservoir in the counter electrode) can be hydrated in situ (i.e., while attached to the body) by absorbing water from the skin (i.e., through transepidermal water loss or sweat) or from a mucosal membrane (e.g., by absorbing saliva in the case of oral mucosal membranes). Once hydrated, the device begins to deliver ionized agent to the body. This enables the drug reservoir to be manufactured in a dry state, giving the device a longer shelf life.
Hydrogels have been particularly favored for use as the drug reservoir matrix and electrolyte reservoir matrix in iontophoretic delivery devices, in part due to their high equilibrium water content and their ability to quickly absorb water. In addition, hydrogels tend to have good biocompatibility with the skin and with mucosal membranes. In spite of these advantages however, hydrogels and other hydrophilic polymer components are difficult to laminate to other components of the delivery system. For example, when utilizing a drug reservoir matrix or an electrolyte reservoir matrix composed of a hydrophilic polymer, the matrix begins to swell as it absorbs water from the skin. In the case of hydrogels, the swelling is quite pronounced. Typically, the drug or electrolyte reservoir is in either direct contact, or contact through a thin layer of an electrically conductive adhesive, with an electrode. Typically, the electrode is composed of metal (e.g., a metal foil or a thin layer of metal deposited on a backing layer) or a hydrophobic polymer containing a conductive filler (e.g., a hydrophobic polymer loaded with carbon fibers and/or metal particles). The electrodes (i.e., either metal electrodes or hydrophobic polymers containing a conductive filler), on the other hand, do not absorb water and do not swell. The different swelling properties of the hydrophilic reservoirs and the electrodes results in shearing along their contact surfaces. In severe cases, the shearing action can result in the complete loss of electrical contact between the electrode and the reservoir resulting in an inoperable device.